The invention relates to a circuit arrangement for igniting and operating a high-pressure discharge lamp, provided with
switching means, inductive means and rectifying means together forming a Buck converter, connected to input terminals for connection to a supply source and output terminals for connection of the lamp, comprising means for supplying the lamp with a current through periodic switching of the switching means alternately into a conducting and a non-conducting state by means of a switch-on and switch-off signal, respectively, PA1 which inductive means comprise a primary and a secondary winding, the secondary winding forming part of an integration network for generating the switch-off signal.
A circuit arrangement of the kind mentioned in the opening paragraph is known from European Patent Application EP-A-0 401 931 whose U.S. equivalent is U.S. Pat. No. 5,068,572. The known circuit arrangement is highly suitable for igniting and operating a high-pressure discharge lamp which forms part of a projection TV installation.
The type of switch mode power supply known as Buck converter is also known under other designations such as downconverter, step-downconverter, inductor-coupled step-downconverter and direct-downconverter. An electrical separation in the form of a transformer may be present between input terminals and output terminals.
The Buck converter operates in a self-oscillatory mode in the known circuit arrangement.
It is possible with the known circuit arrangements to supply a substantially constant power to the connected lamp over a comparatively wide current and voltage range, so that a very constant luminous flux is generated by the lamp. The self-oscillatory mode is characterized by low switching losses in the periodic switching of the switching means, especially in the current-voltage range where lamp operation is stable. Preferably, the downconverter is so dimensioned that switching from the non-conducting to the conducting state takes place with a frequency above the limit of human hearing during stable lamp operation. This also has the result that the dimensions of the inductive means can remain comparatively small.
For generating the switch-off signal, a comparison is made in the known circuit arrangement between a separately set control signal and a signal prevalent at an output of the integration network and proportional to the value of the current through the inductive means. The moment the signal at the output of the integration network becomes equal to the separately set control signal, the switch-off signal is generated, whereupon the switching means are switched from the conducting to the non-conducting state.
In the known circuit arrangement, the integration network comprises capacitive means, and the voltage across the capacitive means forms the signal which is proportional to the current value through the inductive means. A controlled current source of comparatively simple construction is realized with the known circuit arrangement by which also the power in a connected load (the lamp) can be controlled. The self-oscillatory mode of the known circuit arrangement is characterized by the fact that the generation of the switch-on signal. The process of switching of the switching means from the non-conducting to the conducting state takes place when the current through the primary winding of the induction means becomes zero. Subsequently a tuned circuit comprising the primary winding of the inductive means is activated. This results in a voltage across the primary winding. The voltage thus generated is transformed to a further secondary winding of the inductive means. The further secondary winding forms a voltage source for generating the switch-on signal. This results in a more complicated circuit construction leading to higher manufacturing cost and making circuit miniaturization more difficult.